The sudden death of 21 polo horses in Florida may have been caused by a toxin that has yet to be identified by tests and could have been in the animals’ feed, vitamins, or supplements, veterinarians said Monday.

The horses from the Venezuelan-owned team Lechuza Caracas became sick just before a tournament Sunday, collapsing and dying on the scene or while being treated at vet clinics or transported, officials said.

Scott Swerdlin, DVM, MRCVS, a veterinarian at Palm Beach Equine Clinic near the polo grounds, treated one of the sick horses. He said it appeared the animals died of heart failure caused by some kind of toxin that could have been in tainted food, vitamins or supplements, or some combination of all three that caused a toxic reaction